Selective sintering is an additive manufacturing or 3D printing technique that typically uses electromagnetic radiation to fuse powders comprising polymers, metals, ceramics, or glass into a mass that has a desired three-dimensional shape. Typically selective sintering techniques start from a digital representation of the 3D object to be formed. Generally, the digital representation is sliced into a series of cross-sectional layers which can be overlaid to form the object as a whole. The selective sintering apparatus uses this data for building the object on a layer-by-layer basis. For example, a laser may selectively fuse powdered material by scanning cross-sections generated from a 3D digital description of an object (for example from a CAD file or scan data) on the surface of a powder bed. Typically, after each cross-section is scanned, the powder bed is lowered by one layer thickness, a new layer of material is applied on top, and the process is repeated until the object is completed. The bulk powder material in the powder bed may be heated to slightly below its melting point, to make it easier for the laser to raise the temperature of the selected regions the rest of the way to the melting point.
Selective sintering can produce parts from a relatively wide range of commercially available powder materials, such as polymers, metals, and composites. Although amorphous and semi-crystalline polymers are both being used in selective sintering, semi-crystalline polymers are preferred because the melting of semi-crystalline polymers typically takes place in a relatively narrow temperature range. This makes it possible to heat the powder bed to a temperature close to the melt temperature, without the risk of unwanted sintering of the powder particles. The energy gap to be bridged with the laser energy is hence minimal, and easier to control. Accordingly, semi-crystalline polymers such as polyamide 12 generally offer a better dimensional accuracy and reproducibility in selective sintering than amorphous polymers such as polycarbonate and polystyrene.
A further parameter which is considered particularly relevant to the accuracy and reproducibility of selective sintering is the undercooling required or afforded by the polymer to recrystallize from the molten state. This is typically represented by ΔT, and may be measured by calorimetry experiments such as DSC. ΔT can be defined as the difference in melting temperature and the crystallization temperature. It is generally accepted that that an increased ΔT results in reduced deformation and curling upon sintering, and therefore is desirable for robust sintering. In view of the above, conventionally there are two main options to reduce problems due to curling and distortion in selective sintering of polymer powders:                a) selecting a polymer with a large ΔT, for example polyamide 12; and/or        b) optimizing the powder bed temperature.        
However, for many applications there is a need of further improvement of selected sintering.